Tips On Mill Foundations — Part II

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Q. Tim, I recall reading your recent article in Powder & Bulk Engineering magazine which talked about some of the items that need to be determined for an engineer to design a silo and foundation. Can you recap some of the specifics, please?

A. S.R., to design a silo that will reliably store and discharge your material over the long term, you need to gather a range of information. Last issue, we discussed material to be stored and required storage capacity. In this second of two parts, we'll cover required openings, peripheral and imposed loads, geographical location, and soil conditions.

Whether your company contracts an independent consulting engineer or a silo supplier to design the silo, the following checklists of critical factors to consider can help you get started.

Required Openings

Wall and roof penetrations need to be defined and accounted for in the design. These include:

• Manways
• Level probes
• Temperature probes
• Truck or rail car drive-thru doors
• Personnel doors or openings for access
• Fill and discharge
• Conveyors
• Explosion relief
• Negative air
• Aeration or fluidization

Peripheral and Imposed Loads

Internal and external loads due to anticipated equipment and structures must be accounted for in the design. Examples include:

• Ladder and stair access
• Piping
• Equipment
• Access or support floors
• Conveyance bridge and tower support
• Temperature and level cable support

Geographical Location

The specific location may affect design parameters such as:

• Building code requirements
• Zoning requirements or limitations which may require a variance
• Wind load requirements
• Snow load requirements
• Seismic requirements
• Frost requirements
• Available space (i.e. footprint and height) for the silo

Soil Conditions

A soil analysis inclusive of borings and a geotechnical report is imperative to finalize the design. The report will typically provide:

• Allowable soil-bearing capacity
• Elevation of groundwater table
• Elevation of boring refusal
• Expected or anticipated total and differential settlement given the anticipated dead and live loads of the silo structure
• Recommendation for ground improvement (i.e. overexcavate and backfill, etc.) if required
• Recommendation of foundation type (i.e. shallow, piling, etc.)

This information will help the engineer determine the best solution for the foundation design.

Besides defining the material to be stored and the operational requirements of the silo, the engineer can work with the owner to define and determine the remainder of the required data. As with any design, the level of success is determined in accurately defining and understanding the design assumptions.

Answered by Tim Lease, a registered professional engineer and CEO of WL Port-Land Systems, Inc. He has 35 years of experience in bulk solids storage and handling.